1. Field of the Invention
This invention relates to reversible printing ribbon drive mechanisms for data printers, such as are found in business machines or the like.
2. Description of the Prior Art
Heretofore, reversible printing ribbon drive mechanisms for business machines, etc., have generally been of the eyelet sensing or tension sensing type. In the eyelet sensing type, an eyelet or similar projection at each end of the ribbon actuates a reversing control device to reverse the direction of ribbon feed when the end of the ribbon on a ribbon supply spool is approached. Although eyelet sensing controls generally work satisfactorily since they can derive a reliable controlling effect from the eyelets, they require that all printing ribbons used in connection therewith be provided with such reversing control eyelets.
In the tension sensing type, reversal of the ribbon feed mechanism is effective upon an increase in tension in the ribbon which normally occurs when the ribbon becomes depleted from the ribbon spool forming the supply spool. Difficulty is encountered in ribbon feed mechanisms of this type when the ribbon must be fed around a tortuous path or through a plurality of ribbon guides, or otherwise when appreciable drag is applied against the ribbon which increases the load against which the ribbon feed mechanism must operate in order to incrementally step the ribbon endwise through its path. Here, the additional, and often erratic, drag encountered by the ribbon may cause premature reversal of the ribbon drive.